Sand slurries are used in a variety of industries including petroleum, pipeline, construction and cleaning. One example of where large amounts of sand slurry are used in hydraulic fracturing for increasing oil and gas production. In a hydraulic fracturing process, a fracturing fluid is injected through a wellbore into a subterranean formation at a pressure sufficient to initiate a fracture to increase oil and gas production. Frequently, particulates, called proppants, are suspended in the fracturing fluid and transported into the fracture as a slurry. Proppants include sand, ceramic particles, glass spheres, bauxite (aluminum oxide), and the like. Among them, sand is by far the most commonly used proppant. Fracturing fluids in common use include various aqueous and hydrocarbon gels. Liquid carbon dioxide and nitrogen gas are also used in fracturing treatments. The most commonly used fracturing fluids are aqueous fluids containing cross-linked polymers or linear polymers to effectively transport proppants into formation. At the last stage of a fracturing treatment, fracturing fluid is flowed back to surface and proppants are left in the created fracture to prevent it from closing back after pressure is released. The proppant-filled fracture provides a high conductive channel that allows oil and/or gas to seep through to the wellbore more efficiently. The conductivity of the proppant pack plays a dominant role in increasing oil and gas production. However it is well known that polymer residues from the fracturing fluid greatly reduce the conductivity of the proppant-pack.
The density of sand is about 2.6 g/cm3 while the density of water is 1 g/cm3. The large density difference between sand and water makes sand settle quickly in water, even under conditions of high water turbulence. Once settled, sand is not easily lifted by the flow of the aqueous liquid in which it has settled.
Conventionally, to make a relatively stable slurry under static or/and dynamic conditions, sand is commonly suspended in a viscoelastic fluid. In viscoelastic fluids, yield stress plays a dominant role in suspending the particles. Yield stress is the minimum shear stress required to initiate flow in a viscoelastic fluid. Basically, the viscosity of the fluid works to slow down the rate of particle settling, while the yield stress helps to suspend the particles. Under dynamic conditions, agitation or turbulence further help stabilize the slurry. Therefore, to make stable and cost-effective sand slurries, conventional methods focus on manipulating the rheological properties of the fluid by adding a sufficient amount of viscosifier, for example, a natural or synthetic polymer, into the slurry. It is not unusual that a polymer is used together with a foaming agent to improve the rheology and to reduce the cost.
In some applications, for example, well cleanout and sand cleanout in pipe lines, where slurries have to be made in situ to carry the sand out, the presence of a viscosifier in the liquid medium normally has detrimental effect. This is mainly due to the fact that turbulent flow plays a critical role in transporting sand in these situations while a viscosifier tends to suppress the turbulence.
Flotation has been used in minerals engineering for the separation of finely ground valuable minerals from other minerals. Crude ore is ground to fine powder and mixed with water, collecting reagents and, optionally, frothing reagents. When air is blown through the mixture, hydrophobic mineral particles cling to the bubbles, which rise to form froth on the surface. The waste material (gangue) settles to the bottom. The froth is skimmed off, and the water and chemicals are removed, leaving a clean concentrate. The process, also called the froth-flotation process, is used for a number of minerals.
The primary mechanism in such a flotation process is the selective aggregation of micro-bubbles with hydrophobic particles under dynamic conditions to life the particles to the liquid surface. The minerals and their associated gangue usually do not have sufficient hydrophobicity to allow bubbles to attach. Collecting agents, known as collectors, are chemical agents that are able to selectively adsorb to desired minerals surfaces and make them hydrophobic to permit the aggregation of the particles and micro-bubbles and thus promote separation. Frothers are chemical agents added to the mixture to promote the generation of semi-stable froth. In the so-called reverse flotation process, the undesired minerals, such as silica sand are floated away from the valuable minerals which remain in the tailings. The reverse flotation of silica is widely used in processing iron as well as phosphate ores.
A wide variety of chemical agents are useful as collectors and frothers for flotation of silica particles. Amines such as simple primary and secondary amines, primary ether amine and ether diamines, tallow amines and tall oil fatty acid/amine condensates are known to be useful collectors for silica particles. It is well established that these chemical compounds strongly adsorb to sand surface and change the sand surface from hydrophilic to hydrophobic. In fact, the reason that these compounds are used as collectors is because of their capability of hydrophobising sand surface to allow form stable sand/bubbles aggregations. The preferred collectors are amine collectors having at least about twelve carbon atoms. These collectors are commercially available from, for example, Akzo Nobel or Tomah Products Inc. Other possible collectors are oleate salts which normally need presence of multivalent cations such as Ca++ or Mg++ to work effectively.
Compounds useful as frothers include low molecular weight alcohols including methyl isobutyl carbinol and glycol ethers.
Such flotation methods are not used in making sand slurries, and especially in making sand slurries for various oil field applications.